AMS-based radiocarbon dating was applied to Medieval lime mortars made from burned shells and aggregate including both shore sediments and neovolcanic rocks. Three mortar samples from the city of Leiden near Amsterdam were prepared using the same kind of acid hydrolysis technique as has been earlier used for dating mortars made from burned marble and limestone. Five consecutive CO2 fractions were collected from each sample to form age profiles as functions of the dissolution progress index. One of the samples, from a brick wall of known age, was taken as a reference from the Pieterskerk church. Two other samples were taken from the Burcht circular stronghold on a former island on the Rhine River. The age of Burcht is less well known; thus, the presented results are a contribution to an ongoing discussion on its history.